The object of the present invention is a method allowing to prepare microgels of controlled size by subjecting a composition mainly consisting of a solution of polymer and of a crosslinking agent to a stress imposed for a defined time.
Fast production of water in oil or gas producing wells has always posed a major problem for petroleum operators:
equipments for water separation and treatment are required and generate costly investments, notably in the field of offshore production,
production of water in wells limits the profitability of hydrocarbon reservoir development,
severe regulations for controlling discharges, notably of industrial waters, require the problem to be solved in the reservoir rather than in terms of surface treatment.
Known solutions for preventing water inflows in a hydrocarbon reservoir are most often non-selective injection methods based on treatment of the reservoir rock by injection of a polymer solution.
The effect of the treatment is directly related to the ratio of the thickness of the adsorbed polymer layer to the mean pore radius in the treated layer.
Within the range of low to medium permeabilities, the thickness of the adsorbed layer can be controlled by adjusting the molecular weight of the polymer used.
Treatment of formations of higher permeability requires greater adsorbed layer thicknesses that cannot be obtained by a polymer alone.
The main advantage of using a polymer alone lies in the fact that well clogging risks are low in relation to the risk linked with the use of gelling systems. However, the intensity of the effect remains limited since it is constrained only by the osmotic expansion of the polymer molecule in the reservoir water.
Crosslinking of the polymer by a gelling agent is the most widely used method for improving the efficiency of polymer injection in order to reduce water inflows. Crosslinked polymers are generally more stable than a polymer alone in severe environments (high salinity, high temperature for example). The method however remains uncertain as a result of a bad control of the crosslinking process, which leads to high risks of oil production reduction and even of total or partiel well clogging.
In fact, gelation tests are very often carried out under static conditions and the results obtained, often limited to gel strength classifications according to visual criteria, are very different from the behaviour of the system under dynamic conditions, i.e. the conditions of injection into the formation by means of a well.
The objective of selective treatment of reservoir formations (relative permeabilities modification) with a polymer/crosslinking agent system is to decrease the mobility of the water in the rock without reducing the mobility of the oil so that hydrocarbon production can continue.
To give a rough estimate, in the case of layers whose permeability ranges for example between 2 darcys and 50 millidarcys, a layer having a thickness respectively ranging between 5 xcexcm and 1 xcexcm has to be formed at the surface of the pores in order to reduce the permeability to allowable limits without layer damage risks due to retention phenomena.
Such layer thicknesses can only be obtained by adsorption of aggregates or microgels whose size can be controlled.